Continuous steam generator with circulating atmospheric fluidised-bed combustion

ABSTRACT

A continuous steam generator with a circulating atmospheric fludized-bed chamber is defined by encircling walls essentially on all sides, comprised of gas-permeable tubular walls provided with essentially vertical tubes, and comprises at least one funnel in its lower region. The turbulence combustion chamber has at one essentially vertically arranged heating surface provided with vertical tubes, said heating surface comprises of a welded tube-web-tube combination, and a water/steam working medium flows through the tubes of the encircling walls and the heating surface. All of the tubes of the encircling walls and the heating surface are embodied as evaporator heating surfaces and are mounted in parallel for the circulation of the entire working medium to be evaporated. In addition, all of the tubes of the encircling walls have an inner smooth surface, and the heating surface extends between the bottom of the combustion chamber or the upper edge of the funnel, and the top of the combustion chamber.

The invention relates to a continuous steam generator having acirculating atmospheric fluidized-bed firing system.

In addition to natural circulation and forced circulation steamgenerators, forced continuous steam generators and continuous steamgenerators used to generate electrical energy by burning, for example,fossil fuels are known. The latter steam generators are used inparticular in modern and/or large electrical power plants. In them, theheat released during the combustion of fuel in the combustion chamber ofthe continuous steam generator is transferred to heating surfaces of thecontinuous steam generator. These heating surfaces have a working mediumflowing by them, and they consist of, for example, combustionchamber-enclosing walls, radiating and/or convective heating surfaces.The working medium is part of a water/steam loop in a steam turbine inwhich it gives off the thermal energy that it has absorbed.

Such continuous steam generators, in which the working medium ispreheated, evaporated, superheated, and, in some cases, temporarilysuperheated, in a single passage through the steam generator, have beenknown for a long time and are optionally equipped with burners to burnfossil fuels. A conventional, coal-dust-fired continuous steam generatorhas been disclosed in the publication “Zwangdurchlaufkessel fürGleitdruckbetrieb mit vertikaler Brennkammerberohrung” [forcedcontinuous boiler for sliding-pressure operation with verticalcombustion chamber tubing], VGB Kraftwerkstechnik 64, no. 4, April 1984,H. Juzi, A. Salem, and W. Stocker. As a rule, the combustionchamber-enclosing walls of the continuous steam generator are made ofwelded pipe-web-pipe evaporator heating surfaces. In order to ensurethat the enclosing pipe walls are adequately cooled, either slopedsmooth pipes (in other words pipes having smooth inner walls that extendat an angle within the enclosing pipe walls), internally ribbed verticalpipes or downcoming/riser pipe systems are used (in other words, theenclosing pipe walls are divided into a plurality of wall sectionsthrough which the fluid flows, one after another; see also FIG. 2 c ofthe publication cited above).

In recent years, continuous steam generators having circulatingfluidized-bed firing systems (CFBFSs) have been designed. As is the casewith all power plant systems fired by fossil fuels, an attempt is madeto reduce the emissions resulted from combustion in order to protect theenvironment. This can be done by increasing the power plant's processefficiency combined with a reduction in the amount of fossil fuel used.A portion of the increase in efficiency is accomplished by generatingsteam at high steam parameters (high steam pressures and temperatures).In order for the power plant units to operate efficiently within a wideload range, the steam generators are operating with sliding pressure. Inorder to meet various requirements (a constant high steam temperature,sliding steam pressure, rapid rates of load changes), only the forcedcontinuous steam generating systems referred to above may be used.

For reasons relating to erosion, the combustion chamber-enclosing wallsof continuous steam generators having circulating fluidized-bed firingsystems cannot be positioned at a slope or angle, as is the case withconventional coal-dust-fired continuous steam generators, but ratherthey must have vertical tubes. Therefore, the circulating fluidized-bedfiring systems were mainly combined with evaporator systems that work onthe principle of natural circulation or forced circulation operation andare therefore equipped with vertically tubular enclosing walls. A smallnumber of circulating fluidized-bed firing systems also generate steamby means of forced-circulation systems, however as a downcoming/riserpipe system with low vapor pressures (for example, the Moabit powerplant). Plans have already been made for using CFBFSs-equipped forcedcontinuous steam generators in the pressure range from 100 to 300 bar sothat they will operate more efficiently—in other words, with less fuel.Because of the necessity of forming combustion chamber-enclosing wallsfrom vertical evaporator tubes, tubes that have ribs on their innersides were proposed for cooling the evaporator walls (see publicationcited above).

In the transition from naturally circulating steam generators to(supercritical) forced continuous steam generations operating at highsteam parameters (typically 250 to 300 bar, 560 to 620° C.) in the powerrange from 300 to 600 MWel, the following problems and disadvantagesoccur in the prior art:

-   -   CFBFSs continuous steam generators that are operated with        sub-critical steam pressures use more fuel in comparison with        supercritical steam pressures with the same steam generator        output, therefore causing more hazardous emissions.    -   In contrast to sloped tubes, vertical-tube-equipped forced        continuous steam generators have the disadvantage that the        number of tubes with a given combustion chamber geometry is        larger and that the mass flow density (which is a measure of        working medium flow in kg per m² flow cross-sectional area and        per second) decreases per tube. In order, nevertheless, to        ensure that the tubes are adequately cooled, tubes having        internal ribs are used, or the individual walls of the        combustion chamber-enclosing walls have serial fluid flow.    -   Distributing the entire evaporator flow to a plurality of walls        connected in series has a number of disadvantages:        -   1) The individual walls must be connected by means of            downcoming tubes        -   2) When the evaporator flow is redistributed, demixing            processes occur (different steam contents), which manifest            themselves at the evaporator outlet as temperature            aberrations, which can result in cracks in the walls as a            result of thermal expansion being prevented.        -   3) Higher pressure loss because of higher mass flow density.    -   Tubes with internal ribs have higher pressure losses due to        friction and have the disadvantage that special manufacturing        techniques are required and that the effort and expense needed        to join the part surfaces is greater.

The object of the invention is therefore to provide a continuous steamgenerator having a circulating atmospheric fluidized-bed firing systemin which the aforesaid disadvantages are avoided and/or the followingcriteria are met.

-   -   Use of more economical and more environmentally friendly        continuous steam generators equipped with CFBFSs in the power        range from approximately 300 to 600 MWel, and in a pressure        range of approximately 100 to 300 bar.    -   Achieving efficient combustion chamber design for such a        continuous steam generator incorporating additional heating        surfaces installed inside or, optionally, outside the combustion        chamber.

The object of the invention referred to above is accomplished by thecharacterizing elements of patent Claim 1.

Preferred embodiments of the invention are found in the dependentclaims.

The solution of the invention provides a continuous steam generatorhaving a circulating atmospheric fluidized-bed firing system that hasthe following advantages:

-   -   As a result of combining the combustion chamber-enclosing walls        and additional heating surfaces located in the combustion        chamber as evaporation heating surfaces and causing the working        medium to flow through these evaporator heating surfaces in        parallel, the fluidized-bed combustion chamber and, thus, also        the continuous steam generator can be configured to be much        lower in terms of its design scope and therefore to be more cost        effective.    -   There are economic advantages of using smooth tubes-in other        words tubes that have smooth interior surfaces—in the enclosing        walls of the continuous steam generator, since they are less        expensive than internally-ribbed tubes and also since no        specially manufactured parts are required. Numerous        manufacturers produce a great variety of smooth tubes, which is        not the case with internally-ribbed tubes.    -   Using smooth tubes in the enclosing walls of the continuous        steam generator results in a lower pressure loss in the        evaporator heating surface compared to an evaporator heating        surface made with tubes having internal ribs.    -   The parallel flow of fluid through the enclosing walls and the        additional heating surfaces disposed in the fluidized-bed        chamber produce economic advantages, since it is not necessary        to install intermediate collectors (blending or        pressure-compensation collectors).    -   Assembling the heating surfaces made from smooth tubes is more        economical (no modification of the internal ribbing is        necessary, thus less tubing wasted in assembly).    -   The length or height of the vertical heating surfaces that are        also located in the fluidized-bed combustion chamber is modified        to match the height and construction (different funnels in the        lower area of the combustion chamber) of the fluidized-bed        combustion chamber. This leads to advantages in the assembly of        the heating surfaces, since they can be efficiently integrated        into the combustion chamber base or into the upper edge of the        funnel, as well as the combustion chamber cover.    -   The heating surfaces that are also located in the fluidized-bed        combustion chambers can be designed as heating surfaces that are        heated on one side and welded together to form boxes, or as        bulkhead heating surfaces that are heated on two sides.    -   The desired mass flow density that is necessary in order to        compensate mass flow and heating differences and to achieve        nearly the same outlet temperatures is accomplished through the        integration of additional heating surfaces.    -   The combustion chamber dimensions (cross section, height) and        the integrated heating surfaces are dimensioned in such a way        that the effective heat flow densities permit the use of        vertical smooth pipes in the enclosing walls when mass flow        densities are small.

As a result of the use of heating surfaces that are heated on bothsides, said heating surfaces may be designed in a simple butadvantageous manner by making flat bulkhead heating surfaces from apipe-web-pipe combination. In a preferred embodiment the tubes of thesebulkhead heating surfaces have an internal ribbing which, with lowermass flow densities and the higher heating (because the heating istwo-sided) reliably cool the heating surfaces. In this case the tubes ofthe enclosing walls can remain smooth tubes.

In one preferred embodiment, the heating surface of the invention isheated on one side and the heating surface that is heated on one side isdesigned with smooth tubes in a preferred embodiment. In this way, asalready described for the smooth tubes in the enclosing wall, anessential economic advantage is achieved, since smooth tubes areessentially less expensive, easier to install, and have a lower pressureloss due to friction.

In a preferred embodiment of the heating surface that is heated on oneside, said heating surface is configured as a box-shaped heating surfacehaving a box-shaped cross section. Because of the box-shaped design, theheating surface has a high degree of stability that permits combustionchambers of relatively large continuous steam generators be equippedwith heating surfaces. In a further, preferred embodiment the crosssection of the box-shaped heating surface is designed to be rectangular.

In order to achieve uniform heating of the working medium within thetubes in the enclosing walls, it is advantageous that said tubesessentially have the same heated length. In order to transfer the sameeffect to the tubes in the heating surfaces, it is also advantageous forthe tubes in the heating surfaces to have the same heated length as thetubes in the enclosing walls.

Examples of the invention are explained in greater detail below on thebasis of the drawing and the description.

The drawing shows:

FIG. 1 a schematic diagram of a continuous steam generator having acirculating atmospheric fluidized-bed firing system in a longitudinalsection,

FIG. 2 a schematic diagram of a fluidized-bed combustion chamber of afluidized-bed continuous steam generator having a combustion chamberfunnel showing in a longitudinal cross section,

FIG. 3 as in FIG. 2, a fluidized-bed combustion chamber having twocombustion chamber funnels (“pant leg”) shown in a longitudinal crosssection,

FIG. 4 schematic diagram of a combustion chamber of a fluidized-bedcontinuous steam generator (having one combustion chamber funnel shownin cross section per Section A-A, of FIG. 2, rotated by 90°,

FIG. 5 schematic diagram of a combustion chamber of a fluidized-bedcontinuous steam generator (with two combustion chamber funnels) in thecross section indicated as Section B-B in FIG. 3, section rotated 90°C.,

FIG. 6 schematic cross section of an alternative box-shaped heatingsurface (box bulkhead) of Detail C and FIGS. 4 and 5,

FIG. 7 schematic diagram of a box-shaped heating surface with avertically aligning transition from the fireproof exterior covering tothe upper membrane tubular wall in a longitudinal section, correspondsto Section A-A in FIG. 8,

FIG. 8 schematic cross section of a box-shaped heating surface shown inSection C-C of FIG. 9,

FIG. 9 schematic longitudinal section of a box-shaped heating surface asshown in Section B-B of FIG. 8.

In the continuous steam generators fired with fossil fuel inconventional power plants, in the prior art, the working medium,normally water/steam, is essentially preheated, vaporized, superheated,and optionally temporarily superheated in one pass through the steamturbine loop. The continuous steam generator including the appurtenantfiring system is described below.

FIG. 1 shows a schematic diagram of a continuous steam generator 1having a circulating fluidized-bed firing system 2 (CFBFS) for burningcoal or other combustible materials. The material that is to be burnedis transported through the feed line 10 into the fluidized-bedcombustion chamber or fluidized-combustion chamber 3 of the continuoussteam generator 1 having a CFBFS. In order to construct thefluidized-bed and to burn the material being fed in in combustionchamber 3, a fluidization gas is directed through the feed line 11,normally the fluidized-combustion chamber 3. The fluidization gas isgenerally air, which therefore is used as the oxidizing agent for thecombustion. The exhaust gas or flue gas that results from the combustionand the solids entrained by the exhaust gas (inert material, ashparticles, and non-combusted materials) are transported out of thecombustion chamber 3 in the upper area via opening 12, and they are fedvia an exhaust gas line 13 to a precipitator, generally a centrifugalprecipitator or cyclone precipitator 14. In the precipitator 14, thesolids present in the exhaust gas are largely separating off andreturned back to the combustion chamber 3 via the return line 15. Thelargely purified exhaust gas is fed via the exhaust gas line 16 to asecond exhaust gas 17 stack in which at least one economizer heatingsurface 18, at least one superheater heating surface 19, and possibly atleast one intermediate superheater surface 20 is provided for furtheruse or for the acceptance of the exhaust gas heat. The cross section ofcombustion chamber 3 generally has a rectangular shape. However, it canalso be round or have a different shape.

FIGS. 2 to 5 show in a longitudinal section as well as in a transversesection the rectangularly formed and essentially vertically disposedfluidized-bed chamber 3 of a continuous steam generator 1. Thecombustion chamber 3 is essentially enclosed on all sides by theenclosing walls 4, whereby the enclosing wall 4 seen from the bottomtoward the top comprises the combustion chamber bottom 4.1, thecombustion chamber side walls 4.2, and the combustion chamber top 4.3.The combustion chamber floor 4.1 is generally configured as a nozzleplate through which the fluidization gas is brought in. FIG. 2 shows acombustion chamber 3 having a simple funnel 6 in the lower area of thecombustion chamber. On the other hand, FIG. 3 is a combustion chamber 3having a dual funnel 7, a so-called “pant leg” design. The combustionchamber enclosing walls 4 are configured as heating surfaces throughwhich the working medium flows, and said heating surfaces are made ofgas-tight membrane walls. Such membrane walls can be assembled by meansof gas-tight welding of a combination of tube-web-tube. As a rule, thetube-web-tube combination comprises tubes 5 whose exteriors are smoothand which are each connected by means of separate webs 21. However, itis also possible that finned tubes, whose outer wall is already equippedwith webs and which are connected to each other, can be used.

The present invention relates to a continuous steam generator 1 having acirculating fluidized-bed firing system 2 characterized by a high output(approximately 300 to 600 MWel) and high steam parameters (about 250 to300 bar pressure and 560 to 620° C.). In order to obtain an efficientcombustion chamber design in this performance range, additional heatingsurfaces 8 must also be installed. For thermal technology reasons(uniform heat absorption) said additional heating surfaces 8 arepreferably disposed within the combustion chamber 3.

The continuous steam generator 1 of the invention having a CFBFS 2required that all tubes 5, 9 in the enclosing wall 4 and the heatingsurfaces 8 lying within combustion chamber 3 be embodied as anevaporator heating surface, and that they be connected in parallel forthe flow of the entire working medium that is to be evaporated, that alltubes 5 in the enclosing walls 4 be equipped with a pipe surface areathat is smooth on the inside, and that the heating surfaces 8 extendbetween the combustion chamber base 4.1 or funnel upper edge 24 and thecombustion chamber cover 4.3. By connecting the heating surfaces 8 andthe heating surface of the enclosing wall 4 of the continuous steamgenerator 1 in parallel, as well as by using both heating surfaces as anevaporator heating surface, one achieves the advantage that, bymodifying the number of heating surfaces 8, the combustion chamber 3 canbe designed to be efficient. In other words, using this measure, one isable to optimize the combustion chamber dimensions; above all the heightof the combustion chamber (the distance between the bottom of thecombustion chamber and the top), can be reduced significantly byincluding the heating surfaces 8. Additionally, the effective heat fluxdensities within the fluidized-bed combustion chamber 3 of thecontinuous steam generator 1 of the invention increase to permit tubesthat have a smooth interior surface to be used for the tubes 5 of theenclosing walls 4 despite the reduced working medium mass flow densitiesof about 400 to 1200 kg/m²s. Because of the reduced working medium massflow densities, an improved natural circulation characteristic isachieved within the evaporator heating surface, which means that in thecase of potential local excess heating, the working medium flow ratealso increases here, so that safe tube cooling is ensured.

The use of tubes 5 having a smooth inner surface, also referred to forshort as smooth tubes, has a number of advantages over tubes havinginner ribs such as are used with low mass flow densities. For one thing,smooth tubes are significantly less expensive than internally ribbedtubes; moreover, they have shorter delivery times, can be supplied insubstantially more different sizes, and are generally more available,since internally ribbed tubes usually are merely available as custommanufactured parts; furthermore, smooth pipes are significantly easierto deal with in assembly. Moreover, smooth tubes have a significantlylower working medium pressure loss due to friction compared withinternally ribbed tubes, which has a positive effect on the uniformdistribution of the working medium among the individual tubes 5, as wellas a reduction of the feed pump capacity of continuous steam generator1.

In order to increase the continuous steam generator process efficiencyand, thus, to reduce the hazardous emissions that are caused by thesteam generator firing system and that are released into the atmosphere,continuous steam generators 1 are being operated with increasingfrequency in the supercritical range-in other words, at a steam pressureof over 220 bar as well as in sliding pressure between the supercriticaland subcritical pressure (the operating pressure of the steam generatorslides within the load range of the continuous steam generator—forexample, between 20 to 100% load). In the case of a continuous steamgenerator operating pressure of, for example, 270 bar at full load, thesteam generator reaches the critical pressure range at a partial load ofabout 70% and is operated subcritically below this partial load—in otherwords, in the partial load range roughly below 70% a 2-phase mixtureoccurs in the evaporator during the evaporating process. The solution inaccordance with the invention referred to above ensures that within thevaporization heating surface (enclosing walls 4 and heating surfaces 8)no demixing of the steam and water occurs. This is further supported bythe advantageous configuration-of the continuous steam generator 1 ofthe invention because the flow of working medium through tubes 5, 9 ofthe enclosing walls 4 and the heating surfaces 8 takes place without theassistance of intermediate collectors.

The additional heating surfaces 8 used in the fluidized-bed combustionchamber 3 are so-called bulkhead heating surfaces. Bulkhead heatingsurfaces are self-contained plate-like heating surfaces (in other words,the individual tubes 9 that are located next to each other are connectedto each other by means of webs 22—a welded tube-web-tube combination-toform a bulkhead), in contrast to bundle-type heating surfaces, which aredesigned in an open configuration (in other words, the individual tubeslocated next to each other are not connected to each other by means ofwebs). The heating surfaces 8 are essentially disposed vertically withinthe combustion chamber 3, and the tubes 9 contained therein also extendin an essentially vertical direction.

In accordance with the invention, and depending on the combustionchamber design, the heating surfaces 8 either extend between thecombustion chamber base 4.1 or between the upper edge of the funnel 24and the combustion chamber cover 4.3. In this way, they, together withthe enclosing wall 4, can be fully used to achieve parallel flow of theentire working medium that is to be vaporized. Thus, the heatingsurfaces 8 begin in the lower area of the fluidized-bed combustionchamber 3, essentially at the combustion chamber base or at the funnellower edge 4.1 in a combustion chamber 3 having a funnel 6 (FIG. 2) anda central position of the heating surfaces 8 within the combustionchamber 3 or on the funnel upper edge 24 in a combustion chamber 3having two funnels 7 (FIG. 3) as well as a centered arrangement of theheating surfaces, and it terminates [sic: they terminate] in the upperarea of the fluidized-bed chamber 3 essentially at the combustionchamber cover 4.3. In order to attach the individual heating surfaces 8,said surfaces may, for example, be welded together with the combustionchamber base 4.1 or the upper edge of the funnel 24 and the combustionchamber cover 4.3. If more than two funnels are to be provided in thelower area of the combustion chamber 3, the heating surfaces 8 can beintegrated into the design in the logically corresponding manner.

The parallel feeding of the heating surfaces as well as of the enclosingwall 4 is carried out by collectors (not shown) by means of which theworking medium that is to be vaporized is fed from below to theaforesaid heating surfaces. If the heating surfaces 8 with a combustionchamber 3 having two funnels 7 as shown in FIG. 3 do not begin until theupper edge of the funnel or at the yoke of the funnel 24, said heatingsurfaces 8 can be supplied with working medium via the funnel enclosingwalls 4. A separate parallel feeding of the heating surfaces 8 is alsopossible.

The heating surfaces 8 may be heated on one or two sides. In the case ofheating surfaces that are heated on two sides or in the case of bulkheadheating surfaces 8, it is advantageous to configure the heating surfaces8 with tubes 9 that have internal ribs in order to ensure reliablecooling of the tube 9 in the partial load range of the continuous steamgenerator 1 and in order to prevent the boiling crises or DNBs(departures from nucleate boiling) and drying or dry out in theevaporator tube, something which could occur as a result of theadditional heating of the heating surface 8 from both sides.

One advantageous embodiment of the solution in accordance with theinvention provides for heating the heating surfaces 8 disposed insidethe fluidized-bed combustion chamber 3 on one side. FIG. 6 shows apreferred embodiment of a heating surface 8 heated on one side. Thisheating surface 8 comprised an inner space 23 on the periphery side, andit is designed in a box shape, which is why the heating surface 8 isalso called a box-shaped heating surface or a box bulkhead(s) 8 in thefurther description. FIG. 6 shows a preferred embodiment of thebox-shaped heating surface 8 having a rectangular cross section. The boxbulkhead 8 of FIG. 6 has four side walls consisting of welded membranetube walls that are welded together at the corners, and the membranetube walls are formed of tubes 9 and webs 22. This results in a boxhaving a tube-web-tube design or combination that is welded together tobe gas tight. Instead of the rectangular design of the box-shapedheating surface 8 shown on the cross-sectional side in FIG. 6, saidheating surface can also be designed with a different cross section—forexample, it can be n-cornered (at least three-cornered), round, etc. Inother words, in this case the inner space 23 that is enclosed by thebox-shaped heating surface 8 has an n-cornered or round cross section.

Because of the vertical arrangement of the heating surfaces 8 and thusalso of the tubes 9 as well as the vertical tubes 5 of the enclosingwalls 4, the tubes 5, 9 provide as few possible locations for corrosiveattack as possible to the upward flowing stream of gas and particlesthat is present in the combustion chamber 3. In order to protect thetubes 5, 9 in the lower area of the combustion chamber or in the funnelarea 6, 7 from the high transverse or turbulence flows of the stream ofgas and particles in the fluidized-bed, said tubes are provided with afire-proof covering 25.

A preferred embodiment of the invention in FIGS. 7 to 9 provides thefollowing: The tubes 9 of the heating surface 8, which is provided witha fire-proof covering 25, and which is located in the combustion chamberfunnel area 6, 7, are bent inward in the transition area 26 between thecovered and the non-covered heating surface area 27 and in the area ofthe inner space 23, and the front edges of the fireproof covering 25 andof the non-covered areas 27 of the heating surfaces 8 are configured ina vertical direction aligned with each other. This measure preventserosion attack points to form in the transition area 26 on the tubes 9for turbulent flows of the gas and particle stream.

As a result of the fireproof covering 25 of the tubes 5, 9 in the funnelarea 6, 7 the lengths of the tubes 5, 9 are essentially equally heatedwithin the combustion chamber 3.

The box-shaped heating surfaces 8, that extend across a length L andacross their cross-section across a width B and a depth T, and in thepreferred embodiment they have dimensions of approximately 1.4 to 4.0 macross the width B, approximately 0.1 to 1.0 m across the depth T, andapproximately 20 to 50 m across the length L. This also permits thecombustion chambers 3 of larger continuous steam generators 1 to beproperly equipped.

The tubes 9 used for the box-shaped heating surfaces 8 possess diametersbetween 20 mm and 70 mm in a preferred embodiment. The manufacturing ofthe box-shaped heating surfaces 8 can be accomplished using the sameconventional materials and manufacturing techniques that are used tomanufacture steam generators.

LIST OF REFERENCE NUMBERS

-   1 Continuous steam generator-   2 Circulating fluidized-bed firing system-   3 Fluidized-bed combustion chamber-   4 Enclosing walls-   4.1 Combustion chamber bottom or funnel lower edge-   4.2 Combustion chamber side wall-   4.3 Combustion chamber top-   5 Tube-   6 Funnel, single-leg-   7 Funnel, pant-leg-   8 Heating surface-   9 Tube-   10 Fuel feed-   11 Fluidization gas feed-   12 Flue gas opening or outlet-   13 Exhaust gas line-   14 Centrifugal precipitator-   15 Return line-   16 Exhaust gas line-   17 Second flue gas stack-   18 Eco heating surface-   19 Superheater heating surface-   20 Temporarily superheated heating surface-   21 Web in enclosing wall-   22 Web in heating surface-   23 Innerspace-   24 Funnel upper edge-   25 Heat-proof covering-   26 Transition area-   27 Uncovered area of the heating surface

1. A continuous steam generator having a circulating atmosphericfluidized-bed firing system, having a fluidized-bed combustion chamber,in which the fluidized-bed combustion chamber is essentially defined onall sides by enclosing walls, having gas-tight tubular walls essentiallycomprising vertical tubes and in the lower area at least one funnel, andthe fluidized-bed combustion chamber is embodied with at least oneessentially vertically disposed heating surface equipped with verticaltubes, whereby the heating surface is comprised of a weldedtube-web-tube combination, and whereby the tubes of the enclosing wallsand the heating surface have a water/steam working medium passingthrough them, wherein all tubes of the enclosing walls and the heatingsurface are configured as an evaporator heating surface, and they areconnected in parallel so that all of the working medium that is to beevaporated can pass through them, all tubes of the enclosing walls areconfigured with a tube surface area that is smooth on the inside, andthe heating surface extends between the bottom of the combustion chamberor the top of the funnel edge and the combustion chamber cover.
 2. Thecontinuous steam generator of claim 1, wherein the flow of working mediathrough the tubes of the enclosing walls and of the heating surface isaccomplished without the aid of intermediate collectors.
 3. Thecontinuous steam generator of claim 1, wherein the heating surface canbe heated on both sides.
 4. The continuous steam generator of claim 3,wherein the inner surfaces of the tubes of the heating surface have asingle- or multiple-pitch helical internal ribbing.
 5. The continuoussteam generator of claim 1, wherein the heating surface is configured sothat it can be heated from one side.
 6. The continuous steam generatorof claim 5, wherein the inner surfaces of the tubes of the heatingsurface have a smooth surface.
 7. The continuous steam generator ofclaim 5, wherein the heating surface has a box-shaped cross section witha width and a depth and on the peripheral side comprises an inner spacethat is closed around its circumference.
 8. The continuous steamgenerator of claim 1, wherein the cross section of the box-shapedheating surface is configured to have at least three-corners or to beround.
 9. The continuous steam generator of claim 1, wherein the crosssection of the box-shaped heating surface is configured to berectangular.
 10. The continuous steam generator of claim 7, wherein thetubes of the box-shaped heating surface, which are provided with afireproof covering in the combustion chamber funnel area are bent outinto the area of the inner space in the transition area between thecovered and non-covered heating surface area, and the front edges of thefireproof covering and of the non-covered area of the heating surfaceare configured so that they align in the vertical direction.
 11. Thecontinuous heat generator of claim 1, wherein the tubes of the enclosingwalls essentially have equal heated lengths.
 12. The continuous heatgenerator of claim 1, wherein the tubes of the heating surfaceessentially have the same heated length as the tubes of the surroundingwalls.
 13. The continuous steam generator of claim 2, wherein theheating surface can be heated on both sides.
 14. The continuous steamgenerator of claim 13, wherein the inner surfaces of the tubes of theheating surface have a single- or multiple-pitch helical internalribbing.
 15. The continuous steam generator of claim 2, wherein theheating surface is configured so that it can be heated from one side.16. The continuous steam generator of claim 15, wherein the innersurfaces of the tubes of the heating surface have a smooth surface. 17.The continuous steam generator of claim 6, wherein the heating surfacehas a box-shaped cross section with a width and a depth and on theperipheral side comprises an inner space that is closed around itscircumference.
 18. The continuous steam generator of claim 2, whereinthe cross section of the box-shaped heating surface is configured tohave at least three-corners or to be round.
 19. The continuous steamgenerator of claim 3, wherein the cross section of the box-shapedheating surface is configured to have at least three-corners or to beround.
 20. The continuous steam generator of claim 4, wherein the crosssection of the box-shaped heating surface is configured to have at leastthree-corners or to be round.